The present invention relates to an electrode plate used in a plasma etching process of semiconductor wafers. More specifically, the invention relates to an electrode plate used in the parallel-plate-type plasma gas etching apparatus, in which a large number of very-small-diameter through holes for enabling transmission of a reaction gas are formed in a disc-shaped electrode plate body.
In the parallel-plate-type plasma gas etching process, an upper electrode plate in which a large number of very-small-diameter through holes for enabling transmission of a reaction gas are formed in a disc-shaped electrode plate body is opposed to a disc-shaped lower electrode plate on which semiconductor wafers are placed. The reaction gas is dissociated by application of a high-frequency power across the two electrode plates, and resultant radicals and ions are vertically made incident on the semiconductor wafers to etch the portions not covered with a photoresist pattern.
Conventionally, an upper electrode plate of the above kind that is made of high-purity glassy carbon is known as a replacement of an electrode plate made of high-density graphite (Japanese Patent Application Unexamined Publication No. Sho. 62-252942].
This electrode plate for plasma etching made of high-purity glassy carbon is produced in the following manner. First, a liquid thermosetting furan resin or phenol resin, a mixed resin thereof, or one of these liquid resins mixed with a powder of the same thermosetting resin is shaped and set into a disc-shaped resin plate having a uniform thickness distribution. Then, the resin plate is fired and carbonized in an inert atmosphere at about 800.degree. C. After being graphitized, when necessary, at a temperature not more than 3,000.degree. C., the resin plate is purified in a deashing furnace. Very-small-diameter through holes are formed in the glassy carbon plate, which is produced by the firing and carbonizing process, by drilling it by, for instance, electric discharge machining. Alternatively, they are formed by drilling during the process of producing the shaped resin plate.
In the above electrode plate for plasma etching, the internal wall surface of the through holes has no pores, i.e., it is a smooth surface. However, it has been found that at the initial stage of the high-frequency power application across the upper and lower electrode plates, plasma hardly develops in a stable manner and, therefore, the charge density varies on the electrode surface.
This will cause an etching rate variation on the semiconductor wafer surface, that is, the semiconductor wafer cannot be etched uniformly. Several times of blank discharging is required to obtain stable plasma. As a result, non-uniformly etched wafers are increased, and the life of the electrode plates is shortened.